The present invention relates generally to the field of thermoelectric devices. More specifically, the present invention relates to thin film thermoelectric devices and methods of manufacturing such devices.
Thermoelectric devices are solid-state devices that convert thermal energy into electrical energy in the presence of a temperature gradient. While the conversion of temperature difference into electricity is due to the Seebeck effect, an inverse reciprocal effect that enables the transfer of heat when electrical energy is provided is known as the Peltier effect. Therefore, a thermoelectric cooling device (also known as a Peltier device) is a solid state heat pump, which transfers heat from one location to another in the presence of an electrical current. In the reciprocal power generation mode, a thermoelectric device can generate electricity if a temperature gradient is applied across it. Thermoelectric devices have tremendous potential in providing eco-friendly solutions to energy and cooling requirements.
Conventional thermoelectric cooling devices use one or more thermoelectric couples in conjunction with a power source for cooling purposes. Typically, such cooling devices have a low cooling density due to their poor material properties, large form factors and soldered interfaces at the cooling boundaries. The cooling power of a thermoelectric cooler is proportional to the power factor P, (P=S2σ, where S is the Seebeck coefficient and σ is the electrical conductivity). In addition, the cooling power of the thermoelectric cooler is inversely proportional to the transport length l. Conventional thermoelectric cooling devices have a long transport length (˜1-3 mm) and low maximum cooling power (˜5 W/cm2). Ideally, a good thermoelectric material should have a large Seebeck coefficient and high electrical conductivity to minimize Joule heating. Additionally, it should have low thermal conductivity to maintain large temperature gradients. These criteria help to define the thermoelectric figure of merit, Z (Z=S2σ/λ, where S is the Seebeck coefficient of material, σ is the electrical conductivity, and λ is the thermal conductivity of the material).
Another parameter for evaluating the performance of thermoelectric materials is a dimensionless quantity defined as ZT. Since the discovery of semiconductors as useful thermoelectric materials in the early 1950s, a large number of materials have been investigated in an attempt to increase the parameter ZT. Among the materials discovered, compound semiconductors, based on Bismuth Telluride (ZT close to 1), are best suited as thermoelectric materials for room temperature applications. Recent breakthroughs in super-lattice and nano-structured materials have resulted in getting high values for ZT, but these are yet to be incorporated in commercial coolers. One of the methods for increasing the ZT of these compound semiconductors involves depositing thin films under suitable conditions. Thin film deposition enables optimization of the relevant parameters. This optimization can be achieved by sequentially growing different thin films of different materials without contaminating the interfaces. Thin film deposition also uses less thermoelectric materials than those used in conventional film deposition, thereby reducing the cost of the thermoelectric devices. Thin film deposition provides flexibility to the process of manufacturing vertical or lateral thermoelectric coolers. Further, lateral thermoelectric coolers are suitable for high cooling densities. Due to short transport lengths, thin film thermoelectric cooling devices have a fast time response, making them suitable for polymerase chain reaction (PCR) and transient cooling applications.
Therefore, thin film thermoelectric cooling devices are more economical, reliable and efficient, as compared to conventional thermoelectric cooling devices. Since the cooling power of the thermoelectric cooler is inversely proportional to the transport length of the cooling elements, thin film thermoelectric elements are suitable for high cooling densities (>100 W/cm2). Removal of a large amount of heat from the cold side of the thermoelectric cooler results in the dissipation of large densities of heat (>200 W/cm2) on the hot side of the thermoelectric cooler. The inability of the thermoelectric cooler to spread or transport heat from the hot side significantly limits the performance of thin film thermoelectric cooling devices. Managing such large densities of heat is the foremost challenge in realizing the true potential of thin film thermoelectric cooling devices.
In the past decade, rapid progress in the field of semiconductor device manufacturing has resulted in a large number of thin film thermoelectric cooling devices being implemented on Silicon (Si) or Gallium Arsenide (GaAs) substrates. However, the ease of processing thermoelectric materials by using standard techniques in the deposition of films on semiconductor substrates is offset by the fact that these films do not spread heat adequately when formed using standard techniques. The process of patterning and etching thermoelectric films usually contaminates the surfaces that are crucial for the performance of these thin film thermoelectric cooling devices. To manage heat densities by using fans and heat sinks for air cooling, it is necessary to fabricate a thin film thermoelectric cooling device with thick thermoelectric legs. Etching thick thermoelectric films consumes a considerable amount of time, involves prolonged exposure to chemicals, and degrades the properties of the films. Since different types of films etch differently, it is difficult, if not impossible, to etch a compound stack of thermoelectric films. Optimization of a thermoelectric film by changing its composition or type requires a new etching process. The restrictions imposed by etching significantly limit the process of material development and incorporation of novel films for the enhanced performance of these thin film thermoelectric cooling devices. The integration steps of etching, patterning, and the like, also result in an increase in the contact resistance and packaging complexity of the thin film thermoelectric cooling devices.
Consequently, there is a need to create a process that incorporates the advantages of thin film thermoelectric materials and also addresses their current drawbacks.